1. Field of the Invention
This invention relates to a magnetic recording medium comprising a base film coated with a magnetic powder mixed with a binder, that is, to magnetic recording tapes suitable for sound or video recording or for use in electronic computers. More particularly, the invention is concerned with a magnetic recording medium which is free of the so-called "squeal" phenomenon, or audible vibrations, of the running tape and is assured of smoothly and stably running properties, especially in environments of high temperature and high humidity.
2. Description of the Prior Art
In general, a magnetic recording tape, when used with a recording-reproducing apparatus such as a cassette tape deck, runs in sliding contact with the magnetic head, tape guides, etc., and therefore it is required to possess a low friction coefficient as well as smoothly and stably running properties. Actually, the tape running past the head, guides, etc. will sometimes stick to and slip on the contacting parts by turns and successively. This alternate stick-and-slip motion may gradually be intensified until longitudinal vibrations of the tape result. These vibrations, known as "stick-slip vibrations", are primarily responsible for the squeal of the tape. The latter phenomenon is not infrequent these days because a recent tendency is toward denser magnetic recording. In case of the magnetic tape, e.g., for audio use, a growing percentage of the products is designed to be low in running speed (4.75 cm/sec) and thinner (6-18.mu.).
In order to secure the running stability of the tape, it has been customary practice to incorporate in the magnetic coating material any of various lubricants including higher fatty acids and their derivatives, liquid paraffin, castor oil, fluorine oil, molybdenum disulfice, and graphite powder, in addition to the binder. However, none of those lubricants have yet imparted adequate lubricity to the tape; they are unsatisfactory in ensuring stable running performance and in preventing the squeal of the tape, particularly in environments of high temperature and high humidity (e.g., at 40.degree. C.-60.degree. C. and 70-90% RH). The present invention has for its object the provision of a magnetic recording medium which overcomes the abovementioned disadvantage of the prior art.